1. Field
This invention relates generally to the art of gas sampling for analysis purposes and more particularly to a novel row average gas sampling apparatus for continuously sampling a stationary source gas stream.
2. Prior Art
Continuous monitoring of the total gaseous mass emissions and individual gas species emissions from stationary sources is becoming ever increasingly important for both process control and compliance with emission standards. Such stationary source emissions, of course, generally constitute high volume gas flows through large ducts. As a consequence, it is impossible or impractical to monitor the total gas flow, and the required emissions data must be obtained by extrapolating data obtained from a relatively small volume representative sample of the total gas flow.
For any given gas concentration distribution in a stationary source effluent gas stream, it is a simple matter to select a sampling array which will yield a representative gas sample suitable for analyses and extrapolation of the resulting emissions data to the total flow volume. Problems arise when more than one species must be sampled and the gas distributions change as a function of time.
One of the greatest difficulties with regard to continuous sampling of a stationary source gas stream, for example, is to effectively handle the temporal changes which occur in the gas concentration distribution since such changes are primarily a function of combustion characteristics and air leakage and hence extremely difficult, if not impossible, to predict. Temporal changes in velocity distribution are not such a problem, since velocity distributions are primarily a function of local flow duct geometry, which is normally fixed, and hence capable of relatively accurate prediction. Another major methodological difficult involved in continuous sampling of a stationary source effluent gas stream involves stratification of both gas velocity and composition.
Current stationary source gas stream sampling techniques for large flow ducts tend to fall into either of two extremes. At one extreme are precision manual tranverse sampling procedures and single point sampling techniques. At the other extreme are flow proportional sampling techniques.
The manual traverse sampling procedures are performed in accordance with the requirements stated in "Standards of Performance for New Stationary Sources," Environmental Protection Agency, set forth in the Federal Register, Vol. 36, No. 159, Part II. This manual sampling procedure, while precise and accurate, is timing consuming, costly, and totally unsuited to continuous monitoring of process streams and stack emissions, for example.
Single point sampling techniques are often utilized for continuous monitoring applications and are characterized by the advantages of relative simplicity and economy. The available single point sampling data for stationary source effluent streams, however, shows stratification levels in the streams on the order of 10 to 15 percent and very poor temporal repeatability. On this basis, single point sampling is undesirable due to accuracy limitations.
As noted earlier, at the opposite extreme of the existing stationary source gas sampling techniques are flow proportional sampling techniques. These sampling techniques require a relatively large multipoint sampling array with means for monitoring or sensing the local gas stream flow velocity at each sampling point. Sample analysis may be accomplished by utilizing either standard wet chemistry techniques or continuous gas analyzers. If wet chemistry techniques are used, the sampling rate at each sampling point must be proportional to the local gas stream flow velocity. If continuous gas analyzers are used, the gas concentration at each sampling point is measured by the corresponding analyzer and then multiplied by the local gas flow velocity to obtain the local mass flow rate. In either event, the flow proportional sampling techniques are complex and costly to the point of being totally impractical for many industrial process control and emission standards compliance applications.
Accordingly, there is a need for an economically feasible and technically acceptable compromise between the above sampling extremes for sampling a stationary source gas stream to derive data which may be extrapolated to determine with acceptable accuracy the total gaseous mass flow or emission and/or the total individual constitutent gas flows or emissions in the gas.